A power amplifier of a transmitter apparatus in a radio communication system is a circuit that consumes the largest amount of power in the entire apparatus, thus the power amplifier has been required to improve its power efficiency. Radio communication systems in recent years transmit a large amount of data, so that linear modulation signals at a high speed and in a broadband are used. Thus a non-linear amplifier of high power-efficiency, such as class C or class D amplifier, is not used. Instead, a liner amplifier of lower power-efficiency, such as class A or class AB amplifier is used with an appropriate margin in back-off (difference between the max. output amplitude level and the saturated output power level).
A smaller back-off for improving the power efficiency sometime increases distortions and widens spectrum of the transmitter apparatus, thereby interfering in an adjacent communication channel.
One of the methods to solve the problem of improving the power efficiency and yet retaining the linearity of a power amplifier is a method of envelope elimination and restoration (EER). This method is disclosed in “Single-sideband transmission by envelope elimination and restoration” written by Kahn, in Proc. IRE, July 1952, page 803-806. According to this method, a transmitter apparatus decomposes a transmission signal into an amplitude component and a phase component, and the phase component, which is to be an envelope signal, is amplified by a non-linear amplifier of high power-efficiency. The power supply of the amplifier is controlled by the amplitude component. The amplitude component and the phase component are thus restructured.
FIG. 7 shows a structure of a transmitter apparatus adopting the method of envelope elimination and restoration. Distributor 302 receives transmission RF signal 301 and distributes it to amplitude limiter 303 and envelope detector 306. Limiter 303 limits the amplitude of the signal distributed by distributor 302 to a bandwidth, thereby obtaining a phase component of transmission RF signal 301. Delay circuit 304 appropriately delays an output of limiter 303.
Power amplifier 305 amplifies an output from delay circuit 304 up to a desirable power level. Envelope detector 306 envelope-detects a signal supplied from distributor 302, thereby obtaining an amplitude component of transmission RF signal 301. Voltage control DC converter 307 outputs a voltage based on a signal supplied from envelope detector 306. Converter 307 controls power amplifier 305 with this voltage.
For instance, when a field effect transistor (FET) is used as power amplifier 305, the voltage supplied from converter 307 controls the drain voltage of power amplifier 305, so that converter 307 modulates the amplitude. The operation discussed above restructures the amplitude component and the phase component of the output from power amplifier 305 into a signal, which is then transmitted from antenna 308.
A method of envelope tracking is known as another method to solve the problem of improving the power efficiency and retaining the linearity, this method is disclosed in “Power amplifiers and transmitters for RF and microwave” written by Raab, F. H.; Asbeck, P; Cripps, S; Kenigton, P. B.; Popovic, Z. B.; Pothecary, N.; Sevic, J. F.; Sokal, N. O.; microwave Theory and Techniques, IEEE Transactions on, Volume: 50 Issue: 3 Mar. 2002, pages: 814-826.
According to this method, an envelope detector detects an amplitude component of a transmission RF signal, and a voltage to be supplied to the power amplifier is controlled in response to the amplitude component detected. The original transmission RF signal, which includes not only a phase component but also an amplitude component, is supplied to the power amplifier, which thus needs to be a linear amplifier.
In the conventional structures discussed above, the voltage control timing needs to be provided exactly to a transmission signal by a delay circuit. FIG. 8A shows a spectrum of a transmission signal having a timing error in voltage control, and FIG. 8B shows a spectrum where no timing error in voltage control is observed.
A timing error produces distortion components 401 as shown in FIG. 8A, and the distortions degrade the performance of the transmission signal as well as interfere in the adjacent channel. No timing errors produce transmission signal 402 free from distortions as shown in FIG. 8B. However, the timing adjustment discussed above is manually done and is a time-consuming job. The timing once adjusted sometimes cannot follow the changes caused by temperature changes or aged deterioration in characteristics of the apparatus.